A photovoltaic cell is a solid state device fabricated from a thin slice of semiconductor material that outputs a relatively low voltage and small amount of current when exposed to light. Many PV cells may be electrically connected together to form a PV module. The current and voltage output from the PV module result from the combined output of the PV cells in the PV module. The PV module protects the PV cells from moisture, contaminants, and damage from flexure and impact, and includes an electrical connector or electrical terminals for electrically connecting one PV module to another or for connecting the PV module to an inverter or other electrical load. PV modules are available with output power of a few tens of watts to a few hundred watts. One or more PV modules may be attached to a support frame and combined with electrical connectors, interconnect cables, and optional components such as temperature sensors and voltage sensors to form a mechanical and electrical assembly referred to as a PV panel. All of the PV modules on one PV panel may be positioned to face in one direction as a group. The PV panel may optionally be placed on a tracking system to follow the sun's diurnal motion. Instead of moving the PV panel, a movable mirror may be directed to reflect sunlight onto all the PV modules on the PV panel. PV panels may be further combined together into an electrical circuit referred to as a PV array for generating larger amounts of electrical power. PV arrays are available with output capacity of a few kilowatts of electric power for residential or small business use, up to hundreds of megawatts for utility-scale generation of electricity.
PV modules include electrical insulation to constrain the flow of electrical energy from PV cells to specified conductive pathways within the modules and to electrically isolate the PV modules from electrically conductive structural elements such as metallic support frames and other external structures. Electrical insulation is also provided on electrical conductors and connectors between PV modules in a PV panel and between PV panels in a PV array for blocking leakage currents to support structures, to the earth (electrical ground reference), to prevent human exposure to hazardous voltages and currents, and to reduce the risk of fire. Materials used for electrical insulation are subject to aging effects, mechanical damage, and damage from exposure to corrosive chemical compounds, any of which may lead to dielectric breakdown of the insulating material and allow potentially damaging or dangerous leakage currents to flow.
Electrical resistance measurements may be made on PV panels to determine the integrity of electrical insulation in the panels and in connections between panels in a PV array. A decrease in the electrical resistance of electrical insulation in a single PV panel can lead to leakage currents which decrease the power output of an entire PV array. Once a leakage current starts to flow, dielectric breakdown can accelerate, so it is important to detect leakage currents quickly so that PV panels with degraded or defective electrical insulation can be isolated from the PV array and repaired.
An insulation resistance tester (IRT) is a measuring instrument which may be used for detecting dielectric breakdown of electrically insulating materials. Some insulation resistance testers, for example the MEGGER™ line of test instruments produced by Megger, Ltd., operate by subjecting components in an electrical circuit to a known, relatively high voltage, and making measurements related to insulation resistance or leakage current. Insulation resistance test procedures may include warnings to test personnel to make sure that the circuit being tested is not energized by power sources other than the test instrument itself, and may warn personnel to avoid hazardous voltages produced during insulation resistance tests. In addition to current and voltage hazards, PV panels may be placed on building roofs or other locations that are difficult or dangerous for test personnel to access for the purpose of conducting insulation resistance tests. Testing insulation resistance of PV panels in a PV array using conventional methods can therefore be a lengthy, labor-intensive activity because each panel or group of panels being tested must be de-energized and electrically isolated, for example by removing electrical cables or wires between the PV panel being tested and the rest of the PV array before testing can be conducted safely and accurately. Any wires or cables removed before testing must then be reinstalled after tests are complete. Alternately, a PV panel to be tested may be mechanically and electrically disconnected from a PV array. Whichever of these methods is used for electrically isolating a PV panel to be tested, some disassembly and reassembly of PV array components may be required, thereby risking personnel exposure to the hazards of an installed and possibly energized PV array. There is also a risk of damage to the PV array components during disassembly and reassembly.
Completing a set of insulation resistance measurements can take an entire PV array off line for the duration of the test. The larger the PV array, the greater the difficulty in identifying and correcting an electrical insulation problem and the greater the economic loss associated with the value of power that would otherwise have been generated during insulation resistance testing. The difficulty and expense in conducting insulation resistance tests by conventional methods creates economic and safety disincentives for regular monitoring of insulation resistance throughout a PV array. If insulation resistance is not monitored sufficiently often, problems with electrical insulation may not be found before a destructive fault occurs.
It would be preferable to conduct insulation resistance tests on every PV panel and associated interconnect cables in a PV array without mechanically removing PV panels or interconnect cables from the PV array and without exposing test personnel to hazardous voltages and currents, either from current and voltage generated by the PV array or from current and voltage injected into the PV array by an insulation resistance tester. It would further be preferable to monitor changes in insulation resistance everywhere in large PV arrays comprising many hundreds or many thousands of PV panels, and to detect dielectric breakdown in electrically insulating materials before the power output of the entire array is affected and before PV array components are damaged.